Due to the deployment of more and more frequency bands for cellular radio-communications applications, the complexity of radio front-end circuitry to be used in such applications increases. Normally, at least one relatively expensive external (or “off-chip”) filter, usually a SAW (Surface Acoustic Wave) filter, has to be used for each frequency band to be received with the cellular radio. Therefore the size and cost of the external front-end components increase when introducing new frequency bands. Striving towards more flexible front-end solutions requires new circuit solutions that can handle strong interferers and prevent harmonic down-conversion without sacrificing any other performance.
It is thus desirable to provide radio front-end circuitry that eliminates the need for at least some of the off-chip filters, e.g. relatively expensive SAW filters, that are normally used in present cellular radio communications circuitry, or that at least relaxes the requirements on such off-chip filters, which in turn facilitates a reduction of manufacturing cost.
US 2009/0195324 A1 discloses an electronic assembly that includes a substrate, a balun transformer formed on the substrate and including a first winding and a second winding and a reaction circuit component formed on the substrate and connected between a center tap of the second winding and ground. The balun transformer and the reaction circuit component jointly form a harmonically suppressed balun transformer having a fundamental frequency, and the reaction circuit component is tuned such that the harmonically suppressed balun transformer resonates at the second harmonic of the fundamental frequency